1. Field of the Invention
The present invention relates to an organic light emitting device, and more particularly, to a white organic light emitting device having a stack structure of blue fluorescence and red/green phosphorescence, in which efficiency of the blue fluorescence is improved to increase lifespan of the white organic light emitting device, color quality is improved, and power consumption is reduced.
2. Discussion of the Related Art
Recently, with the advent of the age of information, a display field that visually displays an electrical information signal has been rapidly developed. In this respect, various flat display devices having excellent performance such as thin profile, lightness in weight, and low power consumption have been developed to quickly substitute for an existing Cathode Ray Tube (CRT).
Examples of the flat display devices include a liquid crystal display device (LCD), a plasma display panel device (PDP), a field emission display device (FED), and an organic light emitting device (OLED).
Of the flat display devices, the organic light emitting device that does not require a separate light source is considered as a competitive application for compact size of the device and display of a clear color.
This organic light emitting device necessarily needs an organic light emitting layer. In the related art, a deposition method using a shadow mask has been used to form the organic light emitting layer.
However, in case of a shadow mask having a large area, sag may occur due to load of the shadow mask. For this reason, it is difficult to use the shadow mask several times, and a defect occurs in forming a pattern of the organic light emitting layer. In this respect, various solutions for substituting for the shadow mask have been suggested. One of the solutions is a white organic light emitting device.
Hereinafter, the white organic light emitting device will be described below.
The white organic light emitting device is characterized in that each layer between a cathode and an anode is deposited without any mask when forming a light emitting diode. In this case, organic layers including an organic light emitting layer are deposited in due order under the vacuum state by varying their components.
The white organic light emitting device has various purpose of uses such as a full color display device having color filter, a backlight of a liquid crystal display device, or a thin light source.
The white organic light emitting device includes a plurality of light emitting layers having different colors. In this case, dopants included in each light emitting layer have different colors. However, a problem occurs in that there is a limitation in components of the dopant included in the corresponding light emitting layer due to properties of the dopant. Also, as the respective light emitting layers are mixed with one another to obtain white light, a peak value is obtained at a wavelength of a color other than red, green and blue, whereby a color reproduction rate is deteriorated when a color filter is provided. Moreover, a problem occurs in that a color shift occurs due to the difference in lifespan among the dopants when the dopants continue to be used.
Moreover, as similar energy levels occur in the boundary between a light emitting layer and a hole transport layer, a problem occurs in that a triplet exciton moves to the hole transport layer over the boundary and thus light emitting efficiency of excited state is deteriorated. In order to solve this problem, an exciton blocking layer (EBL) may be provided. In this case, driving voltages and process steps are increased and lifespan is reduced. This could lead to many problems in obtaining a white organic light emitting device having proper efficiency.
The white organic light emitting device according to the related art includes light emitting layers which are deposited with either a fluorescent structure or a stack structure of blue fluorescent layer and red/green phosphor layers in view of lifespan and power consumption. In this case, the two structures have a common feature in that a blue fluorescent layer of a singlet exciton is used.
In view of the recent technical level, efficiency of the blue phosphor layer has reached a satisfactory level. However, improvement is still required in view of lifespan. For this reason, a white organic light emitting device having a blue fluorescent layer has been mainly developed but is restrictive due efficiency problem in view of color temperature, lifespan and power consumption. In particular, in order to solve this problem, a method for improving efficiency and lifespan of a blue fluorescent layer having a restrictive internal quantum efficiency rate of 25% is required.